{"id":24066,"date":"2021-09-19T22:14:35","date_gmt":"2021-09-20T03:14:35","guid":{"rendered":"https:\/\/www.realclimate.org\/?p=24066"},"modified":"2022-03-12T18:33:04","modified_gmt":"2022-03-12T23:33:04","slug":"the-definitive-co2-ch4-comparison-post","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2021\/09\/the-definitive-co2-ch4-comparison-post\/","title":{"rendered":"The definitive CO2<\/sub>\/CH4<\/sub> comparison post"},"content":{"rendered":"
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There is a new push <\/a>to reduce CH4<\/sub> emissions as a possible quick ‘win-win’ for climate and air quality. To be clear this is an eminently sensible idea<\/a> – as it has been for decades (remember the ‘Methane-to-markets’ initiative<\/a> from the early 2000s?), but it inevitably brings forth a mish-mash of half-remembered, inappropriate or out-of-date comparisons<\/a> between the impacts of carbon dioxide and methane. So this is an attempt to put all of that in context and provide a hopefully comprehensive guide to how, when, and why to properly compare the two greenhouse gases.<\/p>\n\n\n\n\n\n\n\n

Historical comparisons <\/strong><\/p>\n\n\n\n

First of all, let’s be clear about the relative magnitude of the gas concentrations. In 2020, CO2<\/sub> was at ~410 parts per million, while CH4<\/sub> was around 1870 parts per billion<\/em> (or 1.87 ppm, a factor of more than 200 smaller). However the relative rise since the pre-industrial is three times larger for CH4<\/sub>, around 150%, compared to the 50% increase in CO2<\/sub>.<\/p>\n\n\n\n

The radiative forcing from these changes in concentrations<\/em> can be easily calculated using standard formulas (from Etminan et al, 2016<\/a><\/span> which supersede the slightly simpler ones from IPCC TAR), as about 2 W\/m2 <\/sup>for the CO2<\/sub> change and 0.65 W\/m2<\/sup> for CH4<\/sub>. <\/p>\n\n\n\n

But methane’s role in atmospheric chemistry and as a source of stratospheric water vapour means that it has a bigger effect on climate than just the direct effect of its concentration. Methane emissions have a feedback on its own lifetime, serve as an ozone precursor, and reduce the production of sulphate and nitrate aerosols (and consequently indirect cloud-aerosol effects), all of which amplify its net warming effect to about 1.2 W\/m2<\/sup> (to about 60% of the CO2<\/sub> effect since 1750). There is also a very small impact of the CH4<\/sub> oxidation to CO2<\/sub> itself for any fossil-fuel derived methane. <\/p>\n\n\n

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(a) Historical radiative forcing by emissions (W\/m2) (IPCC fig TS.15a).(b) Historical concentrations of CO2, CH4<\/sub> and N2<\/sub>O (IPCC AR6 fig TS.9b)<\/figcaption><\/figure>\n<\/div>\n\n\n

This implies that if you convert the impacts of each set of emissions into temperatures, as was done in the IPCC AR6 report, you get about 0.75\u00baC from the changes in CO2<\/sub> and 0.5\u00baC for CH4<\/sub> (from the late 19th C, see figure below) or 1\u00baC and 0.6\u00baC, respectively, from 1750. Thus despite the smaller concentrations and changes in methane compared to carbon dioxide, the impacts are comparable.<\/p>\n\n\n\n

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Historical contributions to global warming by emissions (IPCC AR6 SPM)<\/figcaption><\/figure><\/div>\n\n\n\n

Stocks and flows<\/strong><\/p>\n\n\n\n

Before we go any further though, we need to understand that the effective perturbation time for CO2<\/sub> and CH4<\/sub> in the atmosphere are very different. CO2<\/sub> emissions embed themselves in the atmosphere\/biosphere\/upper-ocean carbon cycle and have very long-term impacts (under natural conditions, some 15% of the CO2<\/sub> perturbation will still be in the atmosphere thousands of years from now). In contrast, methane has a perturbation time-scale of about 12 years. This implies that the impact of CO2<\/sub> on temperature is cumulative (a function of the total emitted CO2<\/sub> or stock<\/em>), while the impact of CH4<\/sub> is a function of current (~decadal) emissions (the flows<\/em>). Stabilizing temperature effects from CO2<\/sub> means getting down to net-zero<\/em> anthropogenic emissions, while stabilizing temperature effects from CH4<\/sub> means simply stabilizing emissions. <\/p>\n\n\n\n

The impacts of emissions of CH4<\/sub> compared to CO2<\/sub> then will have a time-varying component. Over a short time, the enhanced effectiveness of methane will be important but on very long time scales the effects of CO2<\/sub> will be dominant. This is the source of the difference between the “Global Warming Potential” (GWP) numbers calculated at 20 years or 100 years which have been used for decades. You might recall that GWP is defined as the ratio on per-kg basis of the temperature impact of other greenhouse gases compared to CO2<\/sub> over a specific time period. But as is clearly stated in AR6, the suitability of comparative emission metrics depends on your end goal or values. <\/p>\n\n\n\n

For instance, if you use GWP-100 to trade off emissions on the way to a temperature stabilization scenario, it simply doesn’t work (since you can’t balance any net CO2<\/sub> emissions with a particular level of CH4<\/sub> emissions – you would need to have constantly decreasing<\/em> CH4<\/sub>). Hence, newer concepts like GWP* have been developed that take that into account. Nonetheless, the UNFCCC (and the EPA) use the GWPs from IPCC AR4 for calculating CO2eq emissions and have not updated them as the science has progressed. <\/p>\n\n\n\n

Forward-facing comparisons<\/strong><\/p>\n\n\n\n

People tend to be most interested in comparisons related to future choices, and it’s worth bearing in mind that while there are many ways to do this, most don’t relate to real choices that people have, nor do they clearly match up with a consistent set of values. I’ll return to that issue below. So let’s go:<\/p>\n\n\n\n